The general purpose of the proposed research is to increase our understanding of the fine structure and function of chromosomes of eukaryotes, and the changes effected therein during growth and differentiation. Meaningful interpretations of chromosome structure will require precise information about linear and lateral patterns of organization in terms of the distribution during somatic and meiotic mitoses of DNA, RNA, histones and non-histone proteins in euchromatin and heterochromatin. The choice of experimental material will not be restricted to either animals or plants, although Drosophila melanogaster commands special attention because it is well known genetically and contains tissues with mitotically-active chromosomes as well as others with giant polytene chromosomes. The latter offer the opportunity to carry out light- and electron-microscopical studies of fine structure at specific loci in normal and structurally rearranged salivary-gland chromosomes. In all of the studies, observational and experimental procedures will be employed (including phase-contrast, fluorescent, electron microscopical, microspectrophotometrical, radioautographical, and biochemical approaches,) but this multipronged continuing effort seems warranted in the light of current discussion about possible methods of genetic engineering (with concern for the control of malignant growth). Previous studies suggest that the program outlined above offers promise of providing new perspectives for the resolution of problems of chromosomal organization in higher plants and animals.